This invention relates to nuclear magnetic resonance, and to coil assemblies for use in nuclear magnetic resonance.
In recent times, a great deal of effort has been concentrated in methods of obtaining N.M.R. spectra from localized regions in space. Such methods are of particular interest in in-vivo spectroscopy. A number of methods have been proposed, involving the application of field gradients, in combination with various sequences of Rf pulses. Such methods are in part successful, but suffer from the disadvantage that the resolution of the spectra obtained tends to be degraded by the use of the gradients.
A number of proposals have been made for localizing N.M.R. spectra, without the use of field gradients. For example, a method has been proposed by M.R. Bendall, (Chemical Physics Letters, 99, 310 (1983)), involving the use of a sequence of pulses of different amplitudes, and phases. For example, three pulses may be used before each signal acquisition, having pulse angles of 2.theta., .theta., 2.theta. respectively, and the experiment is repeated for various combinations of the phase of the first and third pulses, specifically, with phase shifts of 180.degree. applied to the first pulse, and of 90.degree., 180.degree., and 270.degree. applied to the third pulse. The signals obtained from the eight individual signal acquisitions carried out with the various combinations of these phase shifts are then averaged, and it is found that the signal obtained from various parts of the sensitive region of the receiving coil cancel, so that the actual signal sensed by the receiving coil is from a region localized in space.
The shorthand notation used to indicate the eight pulses in this sequence is as follows: EQU 2.theta.[.+-.x]; .theta.; (2.theta.[.+-.x, .+-.y])n
where .theta. represents an arbitrary pulse angle of the Rf pulse used, .+-. indicates that the corresponding pulse is repeated with a 180.degree. phase shift, and .+-.x.+-.y indicates that the corresponding pulse is repeated with the three 90.degree. phase shifts (ie four pulses in all). Pulse sequences of this kind are referred to herein as "depth" pulse sequences.
Various combinations of pulse angles and phase shifts are disclosed in the above papers, and in a number of other papers by Bendall and others.
In particular, the above paper by M. R. Bendall discloses that an improvement in spatial localization can be obtained by using separate Rf coils as transmitting and receiving antennae. A practical difficulty however with the use of separate coils is that, because of their close proximity to each other, the two coils couple together, and each interferes with the tuning of the other. Thus, some method of tuning and detuning the coils, separately from each other, must be devised.
Crossed diodes in series with the transmit coil, and in parallel with the receive coil, have been tried successfully as a method of tuning/detuning, but routine use of this method is presently prevented by the lack of a commercial supply of suitable non-magnetic diodes.